1. Field of the Invention
This invention relates generally to electrostatic bonding of a cover glass to semiconductor cells. In particular, this invention relates to electrostatic bonding techniques which are combined with localized heating to provide bonding of cover glasses to semiconductor cells having metallic conductors placed on the surfaces thereof.
2. Description of the Prior Art
Cover glasses have been bonded to the surface of silicon solar cells in a variety of ways, including electrostatically induced bonding. Recent improvements in silicon solar cells have produced very thin, fragile plates of silicon material which are not readily adaptable to being sealed by the electrostatic bonding techniques of the prior art.
A typical silicon solar cell is about twelve mils thick and two centimeters square. The twelve mil thick substrate of a given material is appropriately doped to produce a pn junction. Photons create hole-electron pairs in the pn junction region and the resultant field causes current flow in the cell. The junction is formed in a thin surface zone of approximately 0.5 microns on the surface of the silicon substrate. Current is removed from the solar cells by means of a grid of very thin metallic conductors deposited on the surface of the cell. The rear surface of the cell has a metallic coating deposited thereupon. The grid-like structure of the metallic conductors permits maximum light energy to impinge upon the solar cell while still providing a means for conducting current out of the cell.
A great number of techniques for bonding glass to metals are known in the prior art. One technique is disclosed in the Pomerantz U.S. Pat. No. 3,397,278 granted Aug. 13, 1968 and assigned to the P. R. Mallory & Co., Inc. A method of encapsulating a planar surface of a silicon semiconductor device with a glass is disclosed which includes heating an insulator such as glass to a temperature below its fusion point to render the glass electrically conductive. An electrical potential is then applied across the juxtaposed glass insulator element and a semiconductor element. An electric current is passed through the points of contact between the elements and creates an electrostatic field between adjoining surfaces causing the juxtaposed elements to be attracted into intimate contact, progressivly closing the gaps therebetween and forming a bond between the adjoining surfaces.
Bonding of a cover glass to a silicon solar cell is described in an article entitled "Integrally Bonded Covers for Silicon Solar Cells" by Allen R. Kirkpatrick, Proceedings of the Eleventh IEEE Photovolatic Specialists Conference, 1975. The method described therein includes positioning a cover glass upon a solar cell surface, raising the temperature of the combination, and applying an electric field to produce a bonding effect. A temperature in excess of 400.degree. C. is used, which is still well below the softening point of the Corning type 7070 glass. This temperature is adequate to create mobile positive ions in the glass. The electric field moves the mobile positive ions away from points of contact at the glass-cell interfaces producing a shallow region depleted of positive ions. Nearly all of the applied voltage, usually several hundred volts, thus appears across the polarized layer thereby formed. A strong electrostatic force proportional to the square of the electric field acts to close the gaps between the glass-cell interface. Starting from a few points of initial contact the entire cell-glass interface is thereby forced together after several minutes. The metalization pattern, or grid, on the front of the silicon cell surface is accomodated by plastic deformation of the glass at a temperature above the strain point of the glass. Such temperatures tend to produce undesirable thermal degradation of the silicon solar cell material. The electrostatic field itself is said to cause plastic flow deformation of the glass around the metallic conductor grids.
Another prior art method requires heating the cover glass and the silicon cell to a temperature near the strain point temperature of the glass, that is, the temperature at which stresses in the glass are thermally relieved in a matter of hours. Mechanical pressure of approximately 150 pounds per square inch pressure is then applied to force the cover glass material around the conductors. This method has disadvantages, one being that gaps are present between the cover glass and the silicon surface alongside the conductors. Another disadvantage is that high internal stresses are produced in the bonded product near the conductors such that with thermal and mechanical cycling the glass cracks, and tiny cracks occur at the silicon glass interface. In addition, this process is applicable to relatively thick cover glasses and silicon solar cells. Thicknesses of 12 mils are required to withstand the mechanical forces required.
Silicon solar cells are now available which have a thickness of two mils and which are as efficient as the older 12 mil cells. Appropriately thinner cover glasses are also available for use in conjunction with the thinner silicon solar cells. These thinner cells and cover glass combinations are particularly useful for applications such as solar satellite power sources where light weight, efficiency and high mechanical reliability are important design requirements. The prior art glass to silicon bonding techniques are not suitable for bonding thin cover glasses to thin silicon solar cells having a metallic grid deposited thereupon.
Prior art electrostatic techniques using conventional heating steps have not adequately solved the problem of providing stress-free, gap-free integral bonds between solar cells and their cover glasses. These conventional heating steps have not provided enough heat to adequately soften the cover glass material near the conductors on the silicon cells so that high stresses are induced in the finished product. On the other hand, the techniques sometimes provide too much heat to the semiconductor material so that thermal degradation occurs. These problems are particularly evident when the thinner cover glasses and shallow junction solar cells are to be bonded.